The present invention relates generally to broadcasting a radio signal. More specifically, a system and method for monitoring broadcast quality and controlling broadcast signal power to achieve a desired signal quality is disclosed.
Whenever a radio signal is broadcast from a transmitter to a remote receiver, the propagation of the signal can vary as a result of various atmospheric effects. Variation in signal propagation due to conditions in the ionosphere is especially significant for radio signals that are refracted from the ionosphere between the transmitter and the receiver.
In many circumstances, companies pay for a certain signal or program to be broadcast into an area. Currently, companies pay for transmission to be made but reliable methods of determining that the transmission was actually received in a given area given the propagation conditions that existed when the transmission was made are not available. Considerable work has been done to monitor broadcasts for the purpose of determining that certain content such as an advertisement that a station has been paid to air is actually aired by the station. Such work is described in U.S. Pat. Nos. 4,025,851, 4,520,404, 4,605,973, 4,739,398, 4,931,871, 4,945,412, 5,436,653, 5,584,050, and 5,892,536. For example, U.S. Pat. No. 4,931,871 describes a method for identification and verification of broadcast program segments involving use of subaudible codes that are mixed with the program segments. In general, such technology focuses on receiving a transmission and analyzing the signal received to determine whether the desired content is included in the signal.
Although work has been done to verify that content that has been paid for is included in a transmitted signal, the problem remains to determine the signal quality of the received transmission at various points within a targeted receiving area. It would be useful if an effective method of gathering data within a target area could be provided so that not only could the transmission of certain content be verified, but the quality of the reception of the content within an area of interest could also be evaluated.
For example, certain broadcasting companies such as the BBC sometimes contract for the broadcast of their signal with an independent transmitter that manages an antenna and provides a signal to the antenna. In such a case, the issue is not the content of the signal transmitted by the antenna, since the content is provided directly to the independent transmitter. The important issue to the broadcasting company is instead the quality of the reception of the signal within a specified receiving area.
In other cases, it is also of interest to know the quality of a signal that is being received in real time. Such information could be used for the purpose of controlling broadcast power while a broadcast is occurring so that only the power required to achieve a desired signal quality within a receiving area of interest is used and excessive broadcast power is not provided to the antenna. Currently, without feedback from a real time measurement of broadcast quality, a larger than required broadcast power must be used that includes extra power to insure that sufficient signal quality is achieved within the receiving area. Power is potentially wasted when signal propagation conditions are particularly good because an excess of power is broadcast to allow for the possibility that propagation conditions are not so favorable.
In some instances, such as broadcasts by the Voice of America, the signal is transmitted to an unfriendly area where it is not practical to provide a monitoring receiver for the purpose of providing broadcast quality feedback. In such a case, it would be useful if some other method of obtaining broadcast quality feedback could be derived so that the power of the signal transmitted into such an area could also be controlled according to existing propagation conditions.
What is needed, therefore, is a method of determining the quality of a signal transmitted into a receiving area both for the purpose of broadcast verification and for real time broadcast signal power control. Preferably, such a method would not necessarily require a feedback monitor located within the receiving area of interest.
A system and method for measuring the quality of a received signal in real time is disclosed. The carrier to noise ratio of a received signal is measured during pauses in the broadcast content such as would normally be expected to occur during human speech. At such times, the strength of the carrier is compared to noise in a sideband to determine the carrier to noise ratio (CNR). In one embodiment, the carrier to noise ratio measurement is synchronized with the occurrence of pauses in the broadcast content to insure that measurements of sideband noise are made when an unmodulated carrier is being broadcast. In another embodiment, a maximum carrier to noise ratio measured within a specified time interval is used to determine the carrier to noise ratio. To avoid measuring an unduly high carrier to noise ratio as a result of signal fading within a sideband, a signal ratio between the carrier and a symmetric portion of the sidebands may be measured.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication lines. Several inventive embodiments of the present invention are described below.
In one embodiment, a method of monitoring the quality of a broadcast includes receiving a broadcast signal at a receiver and measuring the power of the received signal in a carrier band. The power of the received signal in a sideband is also measured. A time is determined at which the ratio of the power of the received signal in the carrier band to the power of the received signal in the sideband is indicative of a carrier to noise ratio of the received signal at the receiver and the carrier to noise ratio of the received signal is stored at the receiver.
In one embodiment, a monitoring receiver is configured to monitor the quality of a broadcast. The monitoring receiver includes a receiving circuit configured to receive a broadcast signal. A power measurement processor is configured to measure the power of the received signal in a carrier band and in a sideband. A selecting processor is configured to determine a time at which the ratio of the power of the received signal in the carrier band to the power of the received signal in the sideband is indicative of a carrier to noise ratio of the received signal at the receiver. A memory is configured to store the carrier to noise ratio of the received signal at the receiver.
In one embodiment, a transmitter is configured to send a synchronization time to a receiver for the purpose of facilitating a signal quality measurement by the receiver. The transmitter includes a transmitting circuit configured to generate a transmission signal to be transmitted to the receiver. A processor is configured to process the transmission signal to determine the synchronization time when a substantially unmodified carrier signal is transmitted. A communication link to the receiver is configured to send the synchronization time to the receiver.
In one embodiment, a transmitter includes a monitoring receiver interface configured to receive a feedback signal from a monitoring receiver, the feedback signal being derived from a measured carrier to noise ratio at the receiver of a signal transmitted by the transmitter. A signal power processor is configured to adjust the power of the transmitted signal using a control scheme to maintain a target carrier to noise ratio at the receiver wherein the feedback signal is used as an input to the control scheme.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.